THE HAEMOCYTES OF INTACT AND INFECTED WITH TREMATODES OF CHINE POND MUSSEL (MOLLUSCA, UNIONIDAE, ANODONTINAE)

Abstract

The hemolymph of Sinanodonta woodiana (Lea, 1834) consists of plasma and cells of four types: prohemocytes (cambial cells), macronucleocytes, basophilic and eosinophilic granulocytes. The three latter cell types derive from the cambial cells though mitosis. The cellular sizes are: prohemocytes 14.3±0.4, basophilic granulocytes 20.9±0.7, young eosinophilic granulocytes 23.8±0.6, older eosinophilic granulocytes 25.7±0.1, macronucleocytes 26.1±0.3. All cells and their nuclei are roundish. Nuclear chromatin is either fine-grained fairly evenly distributed in the karyoplasm (in basophilic granulocytes), or more or less grouped dark-colored small (2-5-6) chromatin blocks. NC-ratio is maximum in macronucleocytes (0.6±0.01). NC-ratio of prohemocytes is 0.4±0.01, that of basophilic granulocytes is 0.3±0.01. NC-ratio of eosinophilic granulocytes is 0.2±0.01. Eosinophilic cells prevail in hemolymph elements: the young eosinophilic granulocytes make up 25.1±0.4 % and the older eosinophilic cells are up to 27.9±0.6 % of all hemocytes. The major functions of hemolymph cells are transport and protection.

Transport is particularly pronounced in basophilic granulocytes in the form of phagocytosis. The protective function of hemocytes of different categories is manifested in a different way. Thus, some of the basophilic granulocytes develop into nephrocytes, which accumulate numerous (11-23) vacuoles of yellow-green-brown color. The vacuoles are subsequently excreted by the mollusk through the kidneys. The protective function of eosinophilic granulocytes is realized as false agglutination (these hemocytes clog into lumps that close wounds), which helps preventing blood loss. Also, eosinophilic granulocytes merge into multinucleated plasmodium, accumulating in large quantities around various foreign bodies (parasites or fragments of destroyed tissues) and encapsulating them to isolate from host tissues. 

At Northern Black Sea Coast, Chinese pond mussel is a common intermediate host of the trematode Rhipidocotyle companula Dujardin, 1845.

The trematode inhabits the mollusk’s gonads. The parasitic sporocysts and cercariae were found in 29 % of examined mollusk specimens. Infestations were weak (up to 10 % of gonads were affected) in 22.4 % of infected mollusks, moderate (10 to 50 % of gonads) in 70 % of contaminated mollusks. Only 7.6 % of infected pond mussels were hyperinfected (100 % of gonads were affected).

Weak trematode infestation is accompanied by localized damage. The total number and volume of parasitic focal lesions are generally insignificant. Moderate infection, and especially a hyperinfection cause not only the lesions in the hostal biotope, but also the overall pathological process in mollusk hosts. Simultaneously, the prohemocytes and basophilic granulocytes as well as their nuclei reduce in size. The total number of prohemocytes also declines by 1.7 times. The greatest decrease in the nuclear-cytoplasmic ratio (by a factor of 1.5-2) is noted also for prohemocytes and basophilic granulocytes. The vacuolization of karyoplasm and cytoplasm, the basophilization of cytoplasm, the degenerative changes in hemocyte nuclei (karyopicnosis, karyorexis, karyolysis), and the total number of aging and dying  hemocytes are directly related to the level of infection intensity.